The invention relates to a machine tool for the machining of shafts and the like, especially for machining of crankshafts. The ends of these crankshafts are secured in respective chucks and two machining units are displaceable in parallel to the longitudinal axis of the workpiece. Each machining unit comprises a ring shaped tool which rotates around the workpiece and which is provided with inwardly directed cutting edges. The tool is secured to the facing side of the tool drum. One of the machining units is provided with an additional workpiece supporting device which rides along with machining unit.
Customarily such machining tools are referred to as milling machines. However, it should be clear that machines of this type may also include grinding machines or the like, whereby the same considerations apply by analogy as for the milling machines.
Machines tools of this type serve especially for the round machining of the bearing pin or journal of the main bearings and of the crank bearings. Such machines are also used for the surface machining of the crankweb and of the counterweight surfaces of forged and cast crankshafts.
Machine tools as described above are known, for example, from a brochure Number 8-4550-11.73, entitled "Machining of the Main and Crank Bearings of Crankshafts by Means of Round Milling Machines", published by the Firm Heller Brothers, Nuertingen. Such machines are also described in another brochure Number Boe203d/12.75, entitled "Production Crankshaft Whirling Machines", published by the Firm Boehringer Brothers of Goeppingen.
In such machines the workpiece support device which is intended for cooperation with one of the two machining units, is arranged in such a manner that the support of the crankshaft is always located directly adjacent to the respective machining point of the respective machining unit. This is necessary to avoid the bending of the crankshaft as a result of the machining forces having regard to the fact that the crankshaft as such has a low flexurals rigidity relative to bending forces.
When the main and stroke bearings are machined simultaneously on such machines, the supporting unit supports the crankshaft at a previously machined main bearing. The one machining unit which is coordinated with the supporting unit machines the crank bearing located to the left of the supporting point, whereas the other machining unit machines the next adjacent main bearing located to the right of the supporting point. Thereafter, both machining units advance to the next bearing pair, whereby the support is applied to the main bearing previously machined. This machining continues correspondingly until the last bearing pair has been machined. Finally, the last crank bearing is machined alone while the last main bearing is supported.
Thus, this type of machining requires as many machining sequences as the number of the crank bearings plus one additional machining sequence for the first main bearing. It is known for an economic machining of crankshafts to first machine all main bearings on a special main bearing machining mill in a single machining operation and to then machine the crank bearings of a machine tool of the above type.
This type of machining results in a substantial increase in the efficiency because only one machining sequence is required for each set of two crank bearings and because only one additional machining sequence is necessary for all main bearings on a special main bearing machine.
However, for the simultaneous machining of two crank bearings, it is necessary for an optimal machining that those crank journals are machined in common which are located on a respective common axis. Stated differently, this means in machines of the Heller System, wherein the crankshaft rotates during the machining, that the rotational feed advance speed is variable in response to the angular positions of the crank pin or journal in order to achieve the most advantageous cutting conditions. In machines in which the crankshaft is stationary and the tool rotates eccentrically about the stationary crankshafts, as in the Heller System, it means, that due to the position of the crank pin center, for machining one must always turn at the elevation of the main bearing axis or at the center elevation of the outer whirling gyro.
For the above reasons it is necessary to machine in common, for example, a six cylinder crankshaft, the crank pins 1 and 6; 2 and 5; as well as 3 and 4, which are always located at a common axis. However, since the additional workpiece support device is coordinated with a certain machining unit, this means that said supporting device supports the crankshaft only adjacent to one of the two crank pins or journals during the machining of the outer crank pin or journal; whereas the other crank pin or journal is subject to being bent out depending on the size of the spacing of the spacing between the other crank pin or journal and the supporting unit. For this reason, the machine tools of the above type have been used so far purely for the crank bearing machining of smaller crankshafts such as four cylinder passenger car crankshafts.